Ink jet printing is a non-impact method that produces droplets of ink that are deposited on a substrate such as paper or transparent film in response to an electronic digital signal. Thermal or bubble jet drop-on-demand ink jet printers have found broad application as output devices for personal computers in the office and the home.
Thermal ink jet printers use a plurality of nozzles each containing a resistor element to fire ink droplets toward the print substrate. Nozzle openings are typically about 25-50 micrometers in diameter. These small openings are easily plugged by precipitating, crystallizing or flocculating materials or by particulate foreign matter. The nozzle openings are exposed to the atmosphere, thereby rendering the ink subject to evaporation or reaction with oxygen or carbon dioxide with the potential to produce particulate, non-dispersed material causing formation of a plug in the nozzle openings. In dye-based inks, evaporation can cause crystallization or precipitation of the dye or solid additives, commonly referred to as "crusting." In pigment-based inks this evaporation can cause precipitation of the dispersant, flocculation of the pigment dispersion, and precipitation of solid additives.
Accordingly, a critical requirement for an ink jet ink is the ability to remain fluid upon exposure to air, so called "decap" conditions. This allows a pen to function after a period of non-use ("long-term decap") or during operation of infrequently utilized nozzles ("short-term decap"). A major concern with all ink jet printers is pluggage of nozzles during operation and between operations. Decap time is a measure of the interval of time that a nozzle can remain exposed to air and continue to print.
Initial evaporation generally causes an increase in viscosity which affects the ability of the nozzle to fire a drop of ink since ink jet pens are designed to operate within specific viscosity ranges. The inception of pluggage may cause distortion of the image, which may appear as a drop of ink which is displaced from its intended position or a splitting of the drop into two or more droplets displaced from the intended target position. In addition, "streamers" or "banners" may appear as artifacts attached to the right side of the alphanumeric characters. On some occasions the drop may reach its intended position but at a lower drop volume producing a lower optical density image. Ultimately the plugged nozzle will fail to fire and no image will be generated.
In a decap test, a series of successive drops are fired at predetermined and increasing time intervals. For example, if the time interval between firings is set at five minutes, then the printings will take place after intervals of 5 minutes, then 10 minutes, then 15 minutes, etc. The interval of time needed to cause failure of the first, fifth and thirty-second consecutively printed drops are recorded. The first drop failure interval is important because it is the critical measure of the reliability of the system without the need for engineering or software cures for printing failure. In addition, it affects the productivity or printing rate because programmed routines must be used to clear the pluggage, so-called "spitting" and these routines interrupt the actual printing chore. The thirty-second drop decap time determines the period of time that a nozzle can remain uncapped and recover after 32 non-printing firings.
Several methods of addressing the crusting problems are known in the art. For example, most ink jet printers are designed to prevent excessive evaporation of solvent from pen nozzles by seating the pen cartridge in an air tight chamber when not in use. These devices become ineffective with continued printer use because dried ink deposits at the rubber seals and the system loses its air-tight condition. Also, it is possible to shut down a printer inadvertently and prematurely, thereby not allowing the printer routine to place the pen nozzles in the air-tight capping chamber.
Another device to combat pluggage is a elastomeric wiper that removes solid formed at the surface of the nozzle. This device is often ineffective because the depth or hardness of the plug resists mechanical removal.
Another pluggage fix is the use of forced air or vacuum suction to clear the nozzle. These devices are often ineffective and add considerable expense to the cost of the printer.
A second critical requirement for inks where the colorant is a pigment is for the pigment dispersion to remain stable throughout the life of the ink jet cartridge. Many cosolvents that impart long decap or rapid penetration are incompatible with the pigment dispersion and therefore cannot be used.
Therefore a need exists for aqueous ink jet inks with good dispersion stability and high resistance to plug formation.